Update GitHub Issue templates (#19723)

Summary:
Now that we have templates for everything we want to handle in the repo, the bot will go back to automatically closing issues that do not use one of the provided templates.

We have an escape valve for generic issues ("For Discussion"), so anyone who gets their issue closed automatically by the bot is unlikely to have read our instructions.
Closes https://github.com/facebook/react-native/pull/19723

Differential Revision: D8435415

Pulled By: hramos

fbshipit-source-id: 41db33cefce1367ad8f3d9440b7bba27565679cb